It is much faster to write to NVRAM (which has write times on the order of approximately 100 microseconds) than to write to disk (which has write time on the order of approximately 10 milliseconds, i.e., 10,000 microseconds). Consequently, storage systems often employ NVRAM to expedite data writes. Thus, when a host writes data to such a storage system, the storage system writes the data into its NVRAM and sends an acknowledgement to the host that the write is complete. Later, in the background, as the NVRAM fills up, the storage system flushes the data in the NVRAM to disk.
In the usual case, an acknowledgement of the write from the storage system to the host is taken by the host as a sign that the written data is durable even though it has not been written to disk. That is, the host assumes, and the storage system is presumed to ensure, that the data in the NVRAM will be preserved even in the event of a system crash or power failure.
To provide this kind of guaranty of data durability, NVRAM devices are often implemented as a peripheral component interconnect (PCI) device containing some dynamic RAM (DRAM) with battery backup. Typically, the battery is able to preserve the data in DRAM for up to several hours or days. There are, however, problems with this approach. For example, any data saved to NVRAM but not yet saved to disk will be lost should the DRAM battery backup expire or fail before power to the storage system is reestablished. Further, the DRAM batteries are generally heavy and unreliable and sometimes need to be replaced.